Composition and method for recovering hydrocarbon fluids from a subterranean reservoir

ABSTRACT

A composition including crosslinked expandable polymeric microparticles capable of hydrolysis at or below neutral pH and a method of modifying the permeability to water of a subterranean formation by introducing such compositions into the subterranean formation. This disclosure further relates to compositions and methods for the recovery of hydrocarbon fluids from a subterranean reservoir or formation subjected to CO2 or CO2 Water Alternating Gas flooding at low pH and increases the mobilization and/or recovery rate of hydrocarbon fluids present in the subterranean formations.

TECHNICAL FIELD

This invention relates generally to compositions and methods for therecovery of hydrocarbon fluids from a subterranean reservoir orformation. More specifically, the invention relates to compositions andmethods for the recovery of hydrocarbon fluids from a subterraneanreservoir or formation subjected to CO₂ or CO₂ Water Alternating Gasflooding. The invention has particular relevance to expandablecrosslinked polymeric microparticle compositions that modify thepermeability of subterranean formations at low pH and increase themobilization and/or recovery rate of hydrocarbon fluids present in thesubterranean formations.

BACKGROUND OF THE INVENTION

In the first stage of hydrocarbon recovery, the sources of energypresent in the reservoir are allowed to move the oil, gas, condensateetc. to the producing wells(s) where they can flow or be pumped to thesurface handling facility. A relatively small proportion of thehydrocarbon in place can usually be recovered by this means. The mostwidely used solution to the problem of maintaining the energy in thereservoir and ensuring that hydrocarbon is driven to the producingwell(s) is to inject fluids down adjacent wells. This is commonly knownas secondary recovery. The fluids normally used are water (such asaquifer water, river water, sea water, or produced water), or gas (suchas produced gas, carbon dioxide, flue gas, and various others). If thefluid encourages movement of normally immobile residual oil or otherhydrocarbon, the process is commonly termed tertiary recovery.

A prevalent problem with secondary and tertiary recovery projectsrelates to the heterogeneity of the reservoir rock strata. The mobilityof the injected fluid is commonly different from the hydrocarbon andwhen it is more mobile various mobility control processes have been usedto make the sweep of the reservoir more uniform and the consequenthydrocarbon recovery more efficient. Such processes have limited valuewhen high permeability zones (commonly called thief zones or streaks)exist within the reservoir rock. The injected fluid has a low resistanceroute from the injection to the production well. In such cases, theinjected fluid does not effectively sweep the hydrocarbon from adjacentlower permeability zones. When the produced fluid is reused, this canlead to fluid cycling through the thief zone to little benefit and atgreat cost in terms of fuel and maintenance of the pumping system. As aresult, numerous physical and chemical methods have been used to divertinjected fluids out of thief zones in or near production and injectionwells. When the treatment is applied to a producing well it is usuallytermed a water (or gas etc.) shutoff treatment. When it is applied to aninjection well it is termed a profile control or conformance controltreatment.

In cases where the thief zone(s) are isolated from the lowerpermeability adjacent zones and when the completion in the well forms agood seal with the barrier (such as a shale layer or “stringer”) causingthe isolation, mechanical seals or “plugs” can be set in the well toblock the entrance of the injected fluid. If the fluid enters or leavesthe formation from the bottom of the well, cement can also be used tofill up the well bore to above the zone of ingress. When the completionof the well allows the injected fluid to enter both the thief and theadjacent zones, such as when a casing is cemented against the producingzone and the cementing is poorly accomplished, a cement squeeze is oftena suitable means of isolating the watered out zone.

Certain cases are not amenable to such methods by virtue of the factthat communication exists between layers of the reservoir rock outsidethe reach of cement. Typical examples of this issue include fractures orrubble zones or washed out caverns existing behind the casing. In suchinstances, chemical gels, capable of moving through pores in reservoirrock have been applied to seal off the swept out zones. When suchmethods fail, the remaining alternatives are to produce the well withpoor recovery rate, sidetrack the well away from the prematurely sweptzone, or abandon the well. Occasionally, the producing well is convertedto a fluid injector to increase the field injection rate above the nethydrocarbon extraction rate and increase the pressure in the reservoir.This can lead to improved overall recovery but note is that the injectedfluid will mostly enter the thief zone at the new injector and is likelyto cause similar problems in nearby wells. All of the above are costlyoptions.

Near wellbore conformance control methods generally fail when the thiefzone is in widespread contact with adjacent hydrocarbon-containing lowerpermeability zones. The reason for this failure is that the injectedfluids can bypass the treatment and reenter the thief zone having onlycontacted a very small proportion, or even none of the remaininghydrocarbon. It is commonly known in the art, that such near wellboretreatments are unsuccessful in significantly improving recovery inreservoirs having crossflow of the injected fluids between zones.

A few processes have been developed with the aim of reducing thepermeability in a substantial portion of the thief zone or at asignificant distance from the injection and production wells. Oneexample of this is the deep diverting gel process is disclosed by Morganet al. (UK Patent Application No. GB 2255360A). This technology has beenused in the field and suffers from sensitivity to unavoidable variationsin quality of the reagents which results in poor propagation. The gelantmixture is a two component formulation and it is believed that thischaracteristic contributed to poor propagation of the treatment into theformation.

The use of swellable cross-linked superabsorbent polymer microparticlesfor modifying the permeability of subterranean formations is disclosedin U.S. Pat. Nos. 5,465,792 and 5,735,349. However, swelling of thesuperabsorbent microparticles described therein is induced by changes ofthe carrier fluid from hydrocarbon to aqueous or from water of highsalinity to water of low salinity. There thus exists an ongoingindustrial need for novel methods to allow efficient propagation throughthe pore structure of hydrocarbon reservoir matrix rock with aparticular need to modify the permeability of subterranean formations atlow pH.

SUMMARY OF THE INVENTION

This invention accordingly provides novel polymeric microparticles inwhich the microparticle conformation is constrained by reversible(labile) internal crosslinks. The microparticle properties, such asparticle size distribution and density of the constrained microparticleare designed to allow efficient propagation through the pore structureof hydrocarbon reservoir matrix rock, such as sandstone, carbonate, andother rocks found in subterranean formations. Unlike previousinventions, these polymers are targeted specifically for reservoirs thathave undergone or are currently undergoing CO₂ for CO₂ Water AlternatingGas (WAG) flooding. The labile crosslinkers were specifically selectedto hydrolyze under low pH conditions allowing the particle to expand byabsorbing the injection fluid (normally water).

The ability of the particle to expand from its original size (at thepoint of injection) depends on the presence of conditions that inducethe breaking of the labile crosslinker. Previous inventions in this areashowed acrylate type labile crosslinkers gives very good performancewhen the reservoir is at or above neutral pH, whereas the presentinvention shows superior performance at or below neutral pH. Theperformance of these particles does not depend on the nature of thecarrier fluid or the salinity of the reservoir water. The particles ofthis invention can propagate through the porous structure of thereservoir without using a designated fluid or fluid with salinity higherthan the reservoir fluid. The expanded particle is engineered to have aparticle size distribution and physical characteristics (e.g., particlerheology) which allows it to impede the flow of injected fluid in thepore structure. In doing so it is capable of diverting chase fluid intoless thoroughly swept zones of the reservoir.

In an aspect, this invention is directed to a composition comprisinghighly crosslinked expandable polymeric microparticles having anunexpanded volume average particle size diameter of from about 0.05 toabout 2,000 microns and a crosslinking agent content from about 50 toabout 200,000 ppm of labile cross linkers and from 0 to about 300 ppm ofnon-labile cross linkers, based on molar ratio of total crosslinkedpolymeric microparticles.

In another aspect, this invention relates to a composition comprisingcrosslinked expandable polymeric microparticles having (i) an unexpandedvolume average particle size diameter from about 0.05 to about 1 micronor from about 0.05 to about 2,000 microns and (ii) a crosslinking agentcontent from about 50 to about 200,000 ppm of at least one labilecrosslinker capable of cleavage (e.g., hydrolysis) at or below neutralpH and from 0 to about 900 ppm of at least one non-labile crosslinker,based on a molar ratio. One or more of the crosslinkers may be amultifunctional crosslinker according to alternative embodiments. In anembodiment, at least a portion of the crosslinked expandable polymericmicroparticles is highly crosslinked.

In another aspect, this invention provides a method of modifying thepermeability to water of a subterranean formation. The method includesintroducing into the subterranean formation a composition comprisingcrosslinked expandable polymeric microparticles having a smallerdiameter than the pores of the subterranean formation and wherein labilecrosslinkers in the crosslinked expandable polymeric microparticlesbreak under the conditions in the subterranean formation to formexpanded polymeric microparticles. In embodiments, from about 100 ppm toabout 10,000 ppm, based on polymer actives and total amount of fluidinjected into the subterranean formation, is added to the subterraneanformation.

It is an advantage of the invention to provide a composition comprisingparticles of low viscosity and optimal size to allow the particles topropagate far from the injection point until encountering a hightemperature zone in the subterranean formation, unlike conventionalblocking agents, such as polymer solutions and polymer gels that cannotpenetrate far and deep into the subterranean formation.

It is another advantage of the invention to provide polymericmicroparticles of a highly crosslinked nature that do not expand insolutions of different salinity resulting in a dispersion that is notaffected by the salinity of the fluid encountered in a subterraneanformation and obviating the need for a special carrier fluid duringtreatment.

It is a further advantage of the invention to provide polymericmicroparticles with a tunable expansion rate based upon the type ofcrosslinkers used and conditions within the subterranean formation.

It is yet another advantage of the invention to provide expandablehighly crosslinked polymeric microparticles with enhanced low pHfunctionality.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other embodiments for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent embodiments do not departfrom the spirit and scope of the invention as set forth in the appendedclaims

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows illustrative examples of labile crosslinkers useful in thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are intended to be clarifying and are notintended to be limiting.

“Amphoteric polymeric microparticle” means a cross-linked polymericmicroparticle containing both cationic substituents and anionicsubstitutents, although not necessarily in the same stoichiometricproportions. Representative amphoteric polymeric microparticles includeterpolymers of nonionic monomers, anionic monomers and cationic monomersas defined herein. Preferred amphoteric polymeric microparticles have ahigher than 1:1 anionic monomer/cationic monomer mole ratio.

“Ampholytic ion pair monomer: means the acid-base salt of basic,nitrogen containing monomers such as dimethylaminoethylacrylate (DMAEA),dimethylaminoethyl methacrylate (DMAEM),2-methacryloyloxyethyldiethylamine, and the like and acidic monomerssuch as acrylic acid and sulfonic acids such as2-acrylamido-2-methylpropane sulfonic acid, 2-methacryloyloxyethanesulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, the like, andcombinations thereof.

“Anionic monomer” means a monomer as defined herein which possesses anacidic functional group and the base addition salts thereof.Representative anionic monomers include acrylic acid, methacrylic acid,maleic acid, itaconic acid, 2-propenoic acid, 2-methyl-2-propenoic acid,2-acrylamido-2-methyl propane sulfonic acid, sulfopropyl acrylic acidand other water-soluble forms of these or other polymerizable carboxylicor sulphonic acids, sulphomethylated acrylamide, allyl sulphonic acid,vinyl sulphonic acid, the quaternary salts of acrylic acid andmethacrylic acid such as ammonium acrylate and ammonium methacrylate,the like, and combinations thereof. Preferred anionic monomers include2-acrylamido-2-methyl propanesulfonic acid sodium salt, vinyl sulfonicacid sodium salt and styrene sulfonic acid sodium salt.2-Acrylamido-2-methyl propanesulfonic acid sodium salt is morepreferred.

“Anionic polymeric microparticle” means a cross-linked polymericmicroparticle containing a net negative charge. Representative anionicpolymeric microparticles include copolymers of acrylamide and2-acrylamido-2-methyl propane sulfonic acid, copolymers of acrylamideand sodium acrylate, terpolymers of acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and sodium acrylate and homopolymers of2-acrylamido-2-methyl propane sulfonic acid. Preferred anionic polymericmicroparticles are prepared from about 95 to about 10 mole percent ofnonionic monomers and from about 5 to about 90 mole percent anionicmonomers. More preferred anionic polymeric microparticles are preparedfrom about 95 to about 10 mole percent acrylamide and from about 5 toabout 90 mole percent 2-acrylamido-2-methyl propane sulfonic acid.

“Betaine-containing polymeric microparticle” means a cross-linkedpolymeric microparticle prepared by polymerizing a betaine monomer andone or more nonionic monomers.

“Betaine monomer” means a monomer containing cationically andanionically charged functionality in equal proportions, such that themonomer is net neutral overall. Representative betaine monomers includeN,N-dimethyl-N-acryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine,N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine,N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine,N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine,N,N-dimethyl-N-acryloxyethyl-N-(3-sulfopropyl)-ammonium betaine,N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine,N-3-sulfopropylvinylpyridine ammonium betaine, 2-(methylthio)ethylmethacryloyl-S-(sulfopropyl)-sulfonium betaine,1-(3-sulfopropyl)-2-vinylpyridinium betaine,N-(4-sulfobutyl)-N-methyldiallylamine ammonium betaine (MDABS),N,N-diallyl-N-methyl-N-(2-sulfoethyl) ammonium betaine, the like, andcombinations thereof. A preferred betaine monomer isN,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine.

“Cationic Monomer” means a monomer unit as defined herein whichpossesses a net positive charge. Representative cationic monomersinclude the quaternary or acid salts of dialkylaminoalkyl acrylates andmethacrylates such as dimethylaminoethylacrylate methyl chloridequaternary salt (DMAEA.MCQ), dimethylaminoethylmethacrylate methylchloride quaternary salt (DMAEM.MCQ), dimethylaminoethylacrylatehydrochloric acid salt, dimethylaminoethylacrylate sulfuric acid salt,dimethylaminoethyl acrylate benzyl chloride quaternary salt (DMAEA.BCQ)and dimethylaminoethylacrylate methyl sulfate quaternary salt; thequaternary or acid salts of dialkylaminoalkylacrylamides andmethacrylamides such as dimethylaminopropyl acrylamide hydrochloric acidsalt, dimethylaminopropyl acrylamide sulfuric acid salt,dimethylaminopropyl methacrylamide hydrochloric acid salt anddimethylaminopropyl methacrylamide sulfuric acid salt,methacrylamidopropyl trimethyl ammonium chloride and acrylamidopropyltrimethyl ammonium chloride; and N,N-diallyldialkyl ammonium halidessuch as diallyldimethyl ammonium chloride (DADMAC). Preferred cationicmonomers include dimethylaminoethylacrylate methyl chloride quaternarysalt, dimethylaminoethylmethacrylate methyl chloride quaternary salt anddiallyldimethyl ammonium chloride. Diallyldimethyl ammonium chloride ismore preferred.

“Cross linking monomer” means an ethylenically unsaturated monomercontaining at least two sites of ethylenic unsaturation which is addedto constrain the microparticle conformation of the polymericmicroparticles of this invention. The level of cross linking used inthese polymer microparticles is high, compared to conventionalsuper-absorbent polymers, to maintain a rigid non-expandablemicroparticle configuration. Cross linking monomers according to thisinvention include both labile crosslinking monomers and non-labilecrosslinking monomers.

“Emulsion,” “microemulsion,” and “inverse emulsion” mean a water-in-oilpolymer emulsion comprising a polymeric microparticle according to thisinvention in the aqueous phase, a hydrocarbon oil for the oil phase andone or more water-in-oil emulsifying agents. Emulsion polymers arehydrocarbon continuous with the water-soluble polymers dispersed withinthe hydrocarbon matrix. The emulsion polymers are optionally “inverted”or converted into water-continuous form using shear, dilution, and,generally an inverting surfactant (See U.S. Pat. No. 3,734,873).

“Ion-pair polymeric microparticle” means a cross-linked polymericmicroparticle prepared by polymerizing an ampholytic ion pair monomerand one more anionic or nonionic monomers.

“Labile cross linking monomer” means a crosslinking monomer which can bedegraded by certain conditions of heat and/or pH, after it has beenincorporated into the polymer structure, to reduce the degree ofcrosslinking in the polymeric microparticle of this invention. Theaforementioned conditions are such that they can cleave bonds in the“crosslinking monomer” without substantially degrading the rest of thepolymer backbone. In embodiments, the labile crosslinker comprises atleast two functional sites. In other embodiments, the labile crosslinkercomprises more than two functional sites. Representative labilecrosslinking monomers that may be used in alternative embodiments of theinvention are shown in FIG. 1. The labile crosslinking monomer ispresent in the crosslinked expandable polymeric microparticles of theinvention in an amount from about 50 to about 200,000 ppm, preferablyfrom about 50 to about 100,000 ppm and more preferably from about 50 toabout 60,000 ppm, based on total weight of crosslinked polymer.

“Monomer” means a polymerizable allylic, vinylic or acrylic compound.The monomer may be anionic, cationic, nonionic, or zwitterionic. Vinylmonomers are preferred, and acrylic monomers are more preferred.

“Nonionic monomer” means a monomer as defined herein which iselectrically neutral. Representative nonionic monomers includeN-isopropylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide,dimethylaminopropyl acrylamide, dimethylaminopropyl methacrylamide,acryloyl morpholine, hydroxyethyl acrylate, hydroxypropyl acrylate,hydroxyethyl methacrylate, hydroxypropyl methacrylate,dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate(DMAEM), maleic anhydride, N-vinyl pyrrolidone, vinyl acetate, andN-vinyl formamide. Preferred nonionic monomers include acrylamide,N-methylacrylamide, N,N-dimethylacrylamide and methacrylamide.Acrylamide is more preferred.

“Non-labile cross linking monomer” means a cross linking monomer whichis not degraded under the conditions of temperature and/or pH whichwould cause incorporated labile cross linking monomer to disintegrate.Non-labile crosslinking monomer is added, in addition to the labilecrosslinking monomer, to control the expanded conformation of thepolymeric microparticle. Representative non-labile crosslinking monomersinclude methylene bisacrylamide, diallylamine, triallylamine, divinylsulfone, diethyleneglycol diallyl ether, the like, and combinationsthereof. A preferred non-labile crosslinking monomer is methylenebisacrylamide. The non-labile crosslinker is present in an amount from 0to about 300 ppm, preferably from about 0 to about 200 ppm, and morepreferably from about 0 to about 100 ppm based on a molar ratio ofcrosslinked polymeric microparticle. In the absence of a non-labilecrosslinker, the polymer particle, upon complete scission of labilecrosslinker, is converted into a mixture of linear polymer strands. Theparticle dispersion is thereby changed into a polymer solution. Thispolymer solution, due to its viscosity, changes the mobility of thefluid in a porous medium. In the presence of a small amount ofnon-labile cross linker, the conversion from particles to linearmolecules is incomplete. The particles become a loosely linked networkbut retain certain “structure.” Such structured particles can block thepore throats of porous media and create a blockage of flow.

In embodiments, the crosslinked expandable polymeric microparticlecomposition of the invention is prepared by free-radical polymerizationfrom about 95 to about 10 mole percent of nonionic monomers and fromabout 5 to about 90 mole percent anionic monomers.

In a preferred embodiment of the invention, the polymeric microparticlesare prepared using an inverse emulsion or microemulsion process toassure certain particle size range. The unexpanded volume averageparticle size diameter of the polymeric microparticle is preferably fromabout 0.05 to about 2,000 microns. In embodiments, the unexpanded volumeaverage particle size diameter is from about 0.05 to about 10 microns.In other embodiments, the unexpanded volume average particle sizediameter is from about 0.1 to about 3 microns, more preferably fromabout 0.1 to about 1 microns.

In an inverse emulsion or microemulsion process, an aqueous solution ofmonomers and crosslinkers is added to a hydrocarbon liquid containing anappropriate surfactant or surfactant mixture to form an inverse monomermicroemulsion consisting of small aqueous droplets dispersed in thecontinuous hydrocarbon liquid phase and subjecting the monomermicroemulsion to free radical polymerization. In addition to themonomers and crosslinkers, the aqueous solution may also contain otheradditives including chelating agents to remove polymerizationinhibitors, pH adjusters, initiators, and other additives. Thehydrocarbon liquid phase comprises a hydrocarbon liquid or mixture ofhydrocarbon liquids. Saturated hydrocarbons or mixtures thereof arepreferred. Typically, the hydrocarbon liquid phase comprises benzene,toluene, fuel oil, kerosene, odorless mineral spirits, the like, andmixtures of any of the foregoing. Surfactants useful in themicroemulsion polymerization process described herein include, forexample, sorbitan esters of fatty acids, ethoxylated sorbitan esters offatty acids, the like, or any mixture or combination thereof. Preferredemulsifying agents include ethoxylated sorbitol oleate and sorbitansesquioleate.

In embodiments, the expandable polymeric microparticle composition ofthe invention comprises at least one of the following properties:anionic, amphoteric, ion-pair, betaine-containing, and combinationsthereof.

Polymerization of the emulsion may be carried out in any manner known tothose skilled in the art. Initiation may be effected with a variety ofthermal and redox free-radical initiators including azo compounds, suchas azobisisobutyronitrile; peroxides, such as t-butyl peroxide; organiccompounds, such as potassium persulfate and redox couples, such assodium bisulfite/sodium bromate. Preparation of an aqueous product fromthe emulsion may be effected by inversion by adding it to water whichmay contain an inverting surfactant.

Alternatively, the polymeric microparticles cross linked with labilecrosslinkers are prepared by internally crosslinking polymer particleswhich contain polymers with pendant carboxylic acid and hydroxyl groups.The crosslinking is achieved through ester formation between thecarboxylic acid and hydroxyl groups. The esterification can beaccomplished by azeotropic distillation (See e.g., U.S. Pat. No.4,599,379) or thin film evaporation technique (See e.g., U.S. Pat. No.5,589,525) for water removal. For example, a polymer microparticleprepared from inverse emulsion polymerization process using acrylicacid, 2-hydroxyethylacrylate, acrylamide and2-acrylamido-2-methylpropanesulfonate sodium as monomer is convertedinto crosslinked polymer particles by the dehydration processesdescribed above.

Representative preparations of cross-linked polymeric microparticlesusing microemulsion process are described in U.S. Pat. Nos. 4,956,400;4,968,435; 5,171,808; 5,465,792; and 5,73,5439.

In embodiments, an aqueous suspension of the polymeric microparticles isprepared by redispersing the dry polymer in water.

In embodiments, this invention is directed to a method of modifying thepermeability to water of a subterranean formation comprising injectinginto the subterranean formation a composition comprising crosslinkedpolymeric microparticles. The microparticles have a crosslinking agentcontent from about 0.9 to about 20 mole percent (50 to 200,000 ppm bymolar ratio of total crosslinked polymer) of labile crosslinkers andfrom 0 to about 300 ppm by molar ratio of total crosslinked polymer ofnon-labile crosslinkers. The microparticles generally have a smallerdiameter than the pores of the subterranean formation and the labilecrosslinkers break under the conditions of temperature and pH in thesubterranean formation to form expanded microparticles. The compositionthen flows through one or more zones of relatively high permeability inthe subterranean formation under increasing temperature conditions,until the composition reaches a location where the temperature or pH issufficiently high to promote expansion of the microparticles. The natureof the crosslinks in the microparticles of the invention result in lowviscosity and optimal size to allow the particles to propagate far fromthe injection point until encountering a high temperature zone in thesubterranean formation, unlike conventional blocking agents, such aspolymer solutions and polymer gels that cannot penetrate far and deepinto the subterranean formation.

Also, the polymeric microparticles of this invention, due to theirhighly crosslinked nature, do not expand in solutions of differentsalinity. Consequently, the viscosity of the dispersion is not affectedby the salinity of the fluid encountered in the subterranean formationand as a result no special carrier fluid is needed for treatment. Onlyafter the particles encounter conditions sufficient to reduce thecrosslinking density, is the fluid rheology changed to achieve thedesired effect.

Among other factors, the reduction in crosslinking density is dependenton the rate of cleavage of the labile crosslinker. In particular,different labile crosslinkers, have different rates of bond cleavage atdifferent temperatures. The temperature and mechanism depend on thenature of the crosslinking chemical bonds. For example, when the labilecrosslinker is PEG diacrylate, hydrolysis of the ester linkage is themechanism of de-crosslinking. Different alcohols have slightly differentrates of hydrolysis. In general, methacrylate esters will hydrolyze at aslower rate than acrylate esters under similar conditions. With divinylor diallyl compounds separated by an azo group such as the diallylamideof 2,2′-Azobis(isbutyric acid), the mechanism of de-crosslinking iselimination of a nitrogen molecule. As demonstrated by various azoinitiators for free radical polymerization, different azo compoundsindeed have different half-life temperatures for decomposition.

Without the intention of being theory bound, in addition to the rate ofde-crosslinking it is believed that the rate of particle diameterexpansion also depends on the total amount of remaining crosslinking. Ithas been observed that the particles expand gradually initially as theamount of crosslinking decreases. After the total amount of crosslinkingpasses below a certain critical density, the viscosity increasesexplosively. Thus, by proper selection of the labile crosslinker, bothtemperature- and time-dependent expansion properties can be incorporatedinto the polymer particles.

The particle size of the polymer particles before expansion is selectedbased on the calculated pore size of the highest permeability thiefzone. The crosslinker type and concentration, and hence the time delaybefore the injected particles begin to expand, is based on thetemperature both near the injection well and deeper into thesubterranean formation, the expected rate of movement of injectedparticles through the thief zone, and the ease with which water cancrossflow out of the thief zone into the adjacent lower permeabilityhydrocarbon-containing zones. A polymer microparticle compositiondesigned to incorporate the above considerations results in a betterwater block after particle expansion, and in a more optimum position inthe formation.

In embodiments of this invention, the composition is added to injectionwater as part of a secondary or tertiary process for the recovery ofhydrocarbon from the subterranean formation. In embodiments, thecomposition is used in a tertiary oil recovery process, one component ofwhich constitutes water injection. In other embodiments of thisinvention, the injection water is added to the subterranean formation ata temperature lower than the temperature of the subterranean formation.

It should be appreciated that this invention has applicability in anysubterranean formation. In an embodiment, the subterranean formation isa sandstone or carbonate hydrocarbon reservoir.

In an embodiment, the diameter of the expanded polymeric microparticlesis greater than one tenth of the controlling pore throat radius of therock pores in the subterranean formation. In another embodiment, thediameter of the expanded polymeric microparticles is greater than onefourth of the controlling pore throat radius of the rock pores in thesubterranean formation.

The present invention has particular applicability for subterraneanformations having a neutral or acidic pH. In an embodiment, the presentinvention is used in a formation having a ph of 7. In other embodiment,the formation has a pH below 7. Preferably, the pH of the formation isin the range of 5-7. In alternative embodiments, the pH of the formationis below 5 or in the range of 4-5.

The polymeric microparticles of the invention may be applied to asubterranean formation as an emulsion, a dry powder, or an aqueoussuspension. In an embodiment, the emulsion is a water-in-oil emulsion.In another embodiment, the aqueous suspension is a concentrated aqueoussuspension.

In embodiments, the composition of the invention comprises an aqueousmedium introduced into the subterranean formation and wherein theaqueous medium includes from about 100 ppm to about 50,000 ppm of thepolymeric microparticles, based on total weight of the aqueous medium.

In embodiments, the composition is added in an amount from about 100 to10,000 ppm, preferably from about 500 to about 1,500 ppm, and morepreferably from about 500 to about 1,000 ppm based on polymer actives,based on total volume of fluid injected into the subterranean formation.

In another embodiment, this invention is directed to a method ofincreasing the mobilization or recovery rate of hydrocarbon fluids in asubterranean formation comprising injecting into the subterraneanformation a composition comprising polymeric microparticles as describedherein. The microparticles generally have a smaller diameter than thepores of the subterranean formation and the labile cross linkers breakunder the conditions of temperature and pH in the subterranean formationto decrease the mobility of the composition.

In an embodiment, the composition of this invention is used in a carbondioxide and water tertiary recovery project. In another embodiment, thecomposition is added to injection water as part of a secondary ortertiary process for the recovery of hydrocarbon from the subterraneanformation.

In another embodiment, the composition and injection water is added to aproducing well. Use of the composition of this invention in a producingwell increases the oil-to-water ratio of the produced fluid. Byinjecting a composition comprising the polymeric microparticles of thisinvention and allowing the particles to expand, the water producingzones can be selectively blocked off.

The foregoing may be better understood by reference to the followingexamples, which are intended for illustrative purposes and are notintended to limit the scope of the invention or its application in anyway.

Example 1

This example illustrates inverse emulsion polymerization techniques forsynthesizing the polymeric microparticle of this invention. Arepresentative emulsion polymer composition was prepared by polymerizinga monomer emulsion consisting of an aqueous mixture of 408.9 g of 50%acrylamide, 125.1 g of 58% sodium acrylamido methylpropane sulfonate(AMPS), 21.5 g water, 0.2 g versene crystals 0.5 g of 1% solution ofmethylenebisacrylamide (MBA). 2.4 g of 5% sodium bromate solution, andvarious levels and types of labile crosslinkers were added to themonomer phase. The monomer phase was dispersed in a mixture of 336 gpetroleum distillate, 80 g ethoxylated sorbitol hexaoleate, and 20 gsorbitan sesquioleate as the continuous phase.

The monomer emulsion was prepared by mixing the aqueous phase and theoil phase. After deoxygenation with nitrogen for 30 minutes,polymerization was initiated by using sodium bisulfite/sodium bromateredox pair at room temperature. The temperature of the polymerization isnot regulated. In general, the heat of polymerization will take thetemperature from about 21° C. to about 94° C. in less than 5 minutes.After the temperature peaked, the reaction mixture was maintained atabout 75° C. for an additional 2 hours.

If desired, the polymeric microparticle can be isolated from the latexby precipitating, filtering, and washing with a mixture of acetone andisopropanol. After drying, the oil and surfactant free particle can beredispersed in aqueous media. Tables 1 and 2 list representativeemulsion polymers prepared according to the method of this example. Thelabile crosslinkers listed in Tables 1 and 2 are shown in FIG. 1.

TABLE 1 Exp. 1 Exp. 2 Exp. 3 Exp. 4 50% Acrylamide 408.9 408.9 408.9408.9 58% Na AMPS 125.1 125.1 125.1 125.1 DI Water 21.57 21.57 21.5721.57 Methylene bisacrylamide (1%) 0.5 0.5 0.5 0.5 Labile crosslinker 172.17 — — — Labile crosslinker 8 — 0.27 — — Labile crosslinker 22 — —0.53 — Labile crosslinker 8 — — — 0.34 Labile crosslinker 21 — — — 3.32Petroleum distillate 336 336 336 336 Ethoxylated sorbitoal hexaoleate 8080 80 80 Sorbitan sesquioleate 20.1 20.1 20.1 20.1

TABLE 2 Exp. 5 Exp. 6 Exp. 7 Exp. 8 50% Acrylamide 408.9 408.9 408.9408.9 58% Na AMPS 125.1 125.1 125.1 125.1 DI Water 21.57 21.57 21.5721.57 Methylene bisacrylamide (1%) 0.5 0.5 0.5 0.5 Labile crosslinker 172.17 — — — Labile crosslinker 8 — 0.27 — — Labile crosslinker 22 — —0.53 — Labile crosslinker 26 — — — 1.67 Petroleum distillate 336 336 336336 Ethoxylated sorbitoal hexaoleate 80 80 80 80 Sorbitan sesquioleate20.1 20.1 20.1 20.1

Example 2

The following brine composition was used to study the expansion of thepolymer particles. The pH of the brine was set to the indicated pH of 3,5, or 6 in Tables 3, 4, and 5, respectively, using acetic acid andNaAcetate buffer.

DI water 85.10 Inverter surfactant 0.66 Sodium thiosulfate 0.15 NaCl10.78 CaCl2 2H2O 0.80 MgCl2 6H2O 0.45 Na2SO4 anhydrous 0.56 Acetic Acid0.50 NaAcetate 1.00 total 100.00

1.82 g of polymer was diluted with 98.18 g of brine solution in a 4 ozsquare bottle. The sample was hand shaken and the viscosity was measured(shown in cP) within the hour to obtain a baseline viscosity reading.All measurements were done with a Brookfiled DV-III ULTRA programmablerheometer at 60 and or 30 rpm using spindle #2c. Samples were placed inan oven at 50 or 70° C. and incubated over several days with 5,000 ppmpolymer actives. Samples were taken out periodically and cooled to roomtemperature and the viscosity was recorded then placed back in the ovenfor further incubation. Table 3-5 show the activation of the polymericmicroparticle using heat. Very minimal expansion was observed with mostsamples in the first 20 days and rapid expansion thereafter. Theparticles used in the experiments of Table 3 (pH=3.0, 50° C.) for Exp.1, 2, 3, 4 correspond to Exp. 5, 6, 7, 8 in Table 2, respectively. Theparticles used in the experiments of Table 4 (pH=5.0, 50° C.) for Exp.5, 6, 7, 8 correspond to Exp. 1, 2, 3, 4, in Table 1, respectively. Theparticles used in the experiments of Table 5 (pH=6.0, 50° C.) for Exp.9, 10, 11, 12 correspond to Exp. 5, 6, 7, 8 in Table 2, respectively.

TABLE 3 (pH 3.0, 50° C.) Day Exp. 1 Exp. 2 Exp. 3 Exp. 4 0 1 0 0 0.5 1 70 0 10 3 10 0 0 18 7 10 0 0 20 12 10 0 0 26 16 10 1 1 32 21 11 1 1 38

TABLE 4 (pH 5.0, 50° C.) Day Exp 5 Exp 6 Exp 7 Exp 8 0 2.5 1 1 0.5 1 142 2.5 1.5 6 18.5 3.5 4 2.5 10 21 3.5 4 2.5 16 24 4 4.5 3 23 29 4.5 5 431 36 8 8.5 5 38 42.5 9.5 11.5 7 52 51 16.5 19.5 13 59 55 20 23 15.5 7756.5 34 32 26 82 56.5 36.5 35.5 30.5 91 58 42.5 40 34.5 98 56 45 39 34.5

TABLE 5 (pH 6.0, 50° C.) Day Exp. 9 Exp. 10 Exp. 11 Exp. 12 0 1 0 0 0 17 0 0 17 3 10 4 6 32 7 13 14 16 36 12 26 26 25 40 16 31 30 27 43 21 3330 27 45

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While this invention may be embodied in many differentforms, there are described in detail herein specific preferredembodiments of the invention. The present disclosure is anexemplification of the principles of the invention and is not intendedto limit the invention to the particular embodiments illustrated. Inaddition, unless expressly stated to the contrary, use of the term “a”is intended to include “at least one” or “one or more.” For example, “adevice” is intended to include “at least one device” or “one or moredevices.”

Any ranges given either in absolute terms or in approximate terms areintended to encompass both, and any definitions used herein are intendedto be clarifying and not limiting. Notwithstanding that the numericalranges and parameters setting forth the broad scope of the invention areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical value, however,inherently contains certain errors necessarily resulting from thestandard deviation found in their respective testing measurements.Moreover, all ranges disclosed herein are to be understood to encompassany and all subranges (including all fractional and whole values)subsumed therein.

Furthermore, the invention encompasses any and all possible combinationsof some or all of the various embodiments described herein. Any and allpatents, patent applications, scientific papers, and other referencescited in this application, as well as any references cited therein, arehereby incorporated by reference in their entirety. It should also beunderstood that various changes and modifications to the presentlypreferred embodiments described herein will be apparent to those skilledin the art. Such changes and modifications can be made without departingfrom the spirit and scope of the invention and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

The claimed invention is:
 1. A method of modifying the permeability towater of a subterranean formation, the method comprising: introducinginto the subterranean formation a composition comprising crosslinkedexpandable polymeric microparticles having an unexpanded volume averageparticle size diameter from about 0.05 to about 2,000 microns, whereinthe microparticles comprise labile internal crosslinks with acrosslinking agent content from about 50 to about 200,000 ppm of atleast one labile crosslinker based on a molar ratio of said polymericmicroparticles, wherein the labile crosslinks are capable of cleavage ator below neutral pH, wherein the microparticles comprise from 0 to about900 ppm of at least one non-labile crosslinker based on a molar ratio ofsaid polymeric microparticles, and wherein the at least one labilecrosslinker is selected from at least one of the following structures1-11, 13, and 15-26:


2. The method of claim 1, wherein the composition comprises an aqueousmedium introduced into the subterranean formation and wherein theaqueous medium includes from about 100 ppm to about 50,000 ppm of saidmicroparticles based on total weight of the aqueous medium.
 3. Themethod of claim 1, wherein the composition is added to injection wateras part of a secondary or tertiary process for the recovery ofhydrocarbon from the subterranean formation.
 4. The method of claim 1,wherein the injection water is added to the subterranean formation at atemperature lower than the temperature of the subterranean formation. 5.The method of claim 1, wherein the diameter of the expanded polymericmicroparticles is greater than one tenth of the controlling pore throatradius of the rock pores in the subterranean formation.
 6. The method ofclaim 1, wherein the subterranean formation is a sandstone or carbonatehydrocarbon reservoir.
 7. The method of claim 1, wherein the compositionis used in a carbon dioxide and water tertiary recovery project.
 8. Themethod of claim 1, wherein the subterranean reservoir was subjected toCO₂ flooding or alternating CO₂ gas and water flooding.